A zero liquid discharge method for co-disposing coal fly ash and flue gas desulfurization (fgd) brines: Zero valent iron reduction and solidification/stabilization

Abstract

The United States coal-fired power industry is facing new challenges related to the management and treatment of coal combustion residues (CCRs) and flue gas desulfurization (FGD) wastewater. The United State Environmental Protection Agency (USEPA) has determined that the wastes generated from the wet FGD systems and coal ash-handling systems contribute the largest proportion of contaminant loading from coal-fired power plants to the environment. The zero-liquid-discharge (ZLD) method by concentrating FGD wastewater into brine and treating the brine with coal fly ash (CFA) through a solidification/stabilization (S/S) process has gained significant interests. The overall objective of this study was to investigate and optimize this ZLD method focusing on two aspects in achieving high efficiency for the immobilization of heavy metals and halides from the FGD brines and CFAs. The first aspect was the utilization of an iron-based reduction process as a pretreatment for the FGD brines to enhance removal of heavy metal oxyanions. Then, the treated FGD brines were applied in the S/S process with CFAs under different conditions and the overall heavy metal and halide immobilization efficiency by the S/S solids was evaluated by different leaching methods. The second aspect was the investigation of the mineral phases in the S/S solids formed and their roles in heavy metal and halide immobilization, and of potential strategies to enhance the formation such mineral phases in the S/S process. To obtain a fundamental understanding of the key mineral, Friedel's salt, in facilitating heavy metal and chloride immobilization, pure form of this mineral was synthesized and studied for its mechanisms in Se(VI) and Cr(VI) uptake under different conditions. The results suggested that aged zero-valent iron (ZVI) was an effective material to remove the heavy metals in the hot FGD brines. High temperature and Mg2+ are the dominant factors that will enhance ZVI's reactivity for the removal of selenate, arsenate, cadmium, and chromate in brine matrices. The pretreatment of brine enhanced the performance of S/S solids made with bituminous CFA (BCFA), but not sub-bitunumous CFA (SCFA) since it already performed well without the pretreatment. Friedel's salt was identified as the key mineral phase which was responsible for the retainment of heavy metal oxyanions and halides. S/S solids made with SCFA contained a higher amount of Friedel's salt because SCFA contained a higher content of lime and reactive aluminate than BCFA. With the same type of CFA, using lime as the activating agent provided more alkalinity than Portland cement, which could further enhance the formation of Friedel's salt. Further enhancing the formation of Friedel's salt in the S/S solids made with BCFA by introducing reactive aluminate and adequate amounts of lime greatly reduced the leaching of Se and chloride. Uptake of Cr(VI) by Friedel's salt was more favorable than Se(VI). Sulfate and carbonate demonstrated a stronger hindering affect than nitrate and chloride for the uptake of both heavy metals by Friedel's salt. Slow transformation from Friedel's salt to stratlingite is a possibility since the CFA can provide reactive silicate and aluminate which are required for this transformation to occur. The results of this work provide valuable insights for the coal-fired power plants in designing and applying related ZLD technologies.